Create 17_Letter_Combinations_of_a_Phone_Number.md

Author: koji

17_Letter_Combinations_of_a_Phone_Number.md

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+## 17. Letter Combinations of a Phone Number
+
+Given a string containing digits from 2-9 inclusive, return all possible letter combinations that the number could represent. Return the answer in any order.
+
+A mapping of digit to letters (just like on the telephone buttons) is given below. Note that 1 does not map to any letters.
+
+
+Example 1:
+
+Input: digits = "23"
+Output: ["ad","ae","af","bd","be","bf","cd","ce","cf"]
+Example 2:
+
+Input: digits = ""
+Output: []
+Example 3:
+
+Input: digits = "2"
+Output: ["a","b","c"]
+ 
+
+Constraints:
+
+0 <= digits.length <= 4
+digits[i] is a digit in the range ['2', '9'].
+
+
+
+```python
+def letterCombinations(self, digits: str) -> List[str]:
+ result = []
+ if digits == "":
+ return result
+ dic = {
+ '2': 'abc',
+ '3': 'def',
+ '4': 'ghi',
+ '5': 'jkl',
+ '6': 'mno',
+ '7': 'pqrs',
+ '8': 'tuv',
+ '9': 'wxyz'
+ }
+ 
+ def combine(build, index):
+ 
+ if index == len(digits):
+ result.append(build)
+ return
+ 
+ chars = dic[digits[index]]
+ for c in chars:
+ combine(build+c, index+1)
+ 
+ combine("", 0)
+ return result
+```
+
+
+```
+Runtime: 32 ms, faster than 51.40% of Python3 online submissions for Letter Combinations of a Phone Number.
+Memory Usage: 14.3 MB, less than 100.00% of Python3 online submissions for Letter Combinations of a Phone Number.
+```